Known electrophotographic image forming apparatuses include a so-called “direct tandem printer”, which is a type of tandem printer that does not employ an intermediate belt transfer system. The direct tandem printer generally includes four photosensitive drums, one for each color, yellow, magenta, cyan and black, a conveying belt for transferring a recording medium, such as a sheet, and four transfer rollers disposed so as to face respective photosensitive drums with the conveying belt between the photosensitive drums and the transfer rollers. A sheet is supplied from, for example, a sheet supply cassette. The sheet is fed onto the conveying belt after the skew of the sheet is corrected by register rollers. While the sheet is fed by the conveying belt between the photosensitive drums and the transfer rollers, toner images formed on each of the photosensitive drums are sequentially transferred onto the sheet.
When the speed of the register rollers conveying a sheet is the same as the speed of the conveying belt conveying a sheet, there likely will be no problems in conveying the sheet. However, it is practically impossible to keep both speeds exactly the same, for example, due to the dimensional tolerances, such as the outside diameters of the register rollers and conveying belt drive rollers. When the speed of the conveying belt conveying the sheet is greater than the speed of the register rollers conveying the sheet, the sheet experiences a tension when contacting both the conveying belt and the register rollers. In this case, the sheet might be pulled with excessive force toward an upstream side in a sheet feeding direction, or the trailing edge of the sheet may be moved or vibrated when the sheet passes through the register rollers and the tension between the conveying belt and register rollers is released. Such vibration in the sheet causes color registration problems. To solve the color registration problems, the speed of the register rollers conveying a sheet is set greater than the speed of the conveying belt conveying a sheet as disclosed in Japanese Laid-Open Patent Publication No. 10-194530.
When the speed of the register rollers conveying a sheet is set greater than the speed of the conveying belt conveying a sheet as disclosed in Japanese Laid-Open Patent Publication No. 10-194530, the sheet experiences slack between the conveying belt and the register rollers. If a sheet fed by the register rollers is conveyed to the conveying belt in a substantially flat position, the sheet does not readily experience slack especially when the sheet is stiff. In this case, the sheet may be pushed or slid over the conveying belt toward a downstream side in the sheet feeding direction by the register rollers.
To make the sheet readily experience slack, it would be helpful if that the sheet were curved while being conveyed between the conveying belt and the register rollers so as to allow the slack in the sheet. However, structures of a first guide member for curving the sheet fed by the register rollers while allowing slack in the sheet do not exist. Therefore, the sheet may not be curved smoothly or readily experience slack due to the stiffness of the sheet. Consequently, improper sheet feeding can occur which can result in damage to the sheet.